The PIs will apply innovations in seismic surface wave tomography to burgeoning broadband seismic data sets in China to construct a 3-D model of radial and azimuthal anisotropy within the crust and uppermost mantle beneath and surrounding much of Tibet. The new tomographic methods include ambient noise tomography and array-based earthquake tomography that promise new structural constraints on isotopic and anisotropic structures from the shallow crust to mantle depths of at least 150 km with a lateral resolution of about 100 km across most of the study region. Targeted data sets leverage past and on-going US PASSCAL experiments by introducing a vast new data resource, the Chinese provincial networks surrounding Tibet. This work will be facilitated by collaboration with researchers at the Institute of Geodesy and Geophysics of the Chinese National Academy of Sciences in Wuhan, China. Preliminary proof-of-concept data processing and inversion efforts establish the quality of the constituent data sets and guarantee the production of integrated isotropic and anisotropic models of the crust and uppermost mantle across all of Tibet.
This work is motivated to gain new insight into fundamental questions related to the continental collision between India and Asia; in particular, the role of the Indian and Asian mantle lithospheres in the collision and how the crust deforms in response to the collision. Detailed consideration of these questions requires high resolution 3D models of isotropic and anisotropic seismic variables in the crust and uppermost mantle across all of Tibet. Anisotropic models especially will provide new constraints on the vertical coherence of strains in the crust that will help resolve debate between competing theories of crustal deformation in response to continental collision.
Collisions between continents are common in earth's history, but at present Tibet is the principal region on the earth that is undergoing this process. Therefore, Tibet presents the best natural laboratory to study continental collisions and to test hypotheses about the resulting deformation of earth's crust. This project is devoted to mapping the middle and deep parts of the crust beneath Tibet using new methods of tomography based on seismic surface waves. This work was conducted in collaboration with Chinese scientists and is based on seismic stations that were installed both by US and Chinese scientists. The crust beneath Tibet is the thickest in the world, up to 80 km which is about double the thickness of the crust beneath the US. We show that the middle crust of Tibet is seismically slow and these low velocity zones exist in a interconnected network that span Tibet, but are slowest in northwest Tibet in a region called the Qiangtang terrane. The middle crust across Tibet is not only slow seismically (seismic shear waves travel very slowly in this region) but it affects seismic waves in a way that depends on the orientation of the wave path and the displacement undergone by the wave. Notably, Love waves (horizontally polarized shear waves) propagate faster than expected due to the Tibetan middle crust and Rayleigh waves (vertically and horizontally polarized surface waves) travel slower, which means that the crust is not only slow it's also anisotropic. We mapped the strength, spatial distribution, and depth extent of this anisotropy and found that it is strongest where seismic waves are slowest, so that the patterns of anisotropy and the low velocity zones in the middle Tibetan crust are related. We interpret these observations as evidence for zones of partial melt with mica-bearing crust across much of Tibet. The existence of partial melt and mica provides new evidence to help geodynamicists and tectonicists interpret the deformation history of this natural laboratory.
